The present invention relates to the field of production of cryogenic and near-cryogenic substances. The invention discloses a method and apparatus for producing a very cold, but not liquid, gas at a predetermined temperature.
Liquid nitrogen is commonly inserted into aluminum cans containing food or beverages, immediately before the cans are sealed. The liquid nitrogen vaporizes and expands by a factor of about 700, creating a large internal pressure, and enabling the aluminum can to withstand the external pressure exerted by other cans in a stack.
Besides adding to the internal pressure, the liquid nitrogen injected into the can serves the purpose of "inerting" the contents of the can. That is, the nitrogen displaces most of the oxygen in the can, due to the rapid vaporization and expansion of the nitrogen in liquid form. Nitrogen is virtually inert at all but the highest temperatures. By eliminating most of the oxygen in the can, and replacing it with nitrogen, the rate of bacterial growth is substantially reduced. In fact, even if oxygen is not actually removed, the addition of nitrogen dilutes the oxygen sufficiently to reduce the effectiveness of the oxygen in supporting the growth of bacteria.
There are applications, however, where the substantial internal pressure due to liquid nitrogen is unnecessary and even undesirable. For example, packages of potato chips or crackers, made of various plastic and/or foil materials, cannot withstand a high internal pressure such as would be produced by vaporization of liquid nitrogen. Yet these and other food products must be capable of being transported and stored, in their containers, for long periods of time, without the growth of harmful bacteria. For this reason, it is desirable to use nitrogen gas which is very cold but not quite liquid. By using a cold gas instead of a liquid, the pressure due to expansion is greatly reduced.
While liquid nitrogen expands by a factor of about 700 when vaporized, cold gaseous nitrogen, say, at -320.degree. F., has an expansion factor of only about 3.8. Cold gaseous nitrogen can therefore be used both to create a modest pressure within a soft container, and to purge unwanted oxygen from its interior.
Various methods of making a cold gas have been described in the prior art. For example, U.S. Pat. No. 3,615,079 describes a heat exchanger wherein a gas to be cooled is bubbled through a liquid heat carrier. U.S. Pat. No. 4,481,780 describes a method for production of a cold gas, the method involving the mixing of a relatively warm gas and a liquid cryogen. U.S. Pat. No. 3,771, 260 discloses a method of combining a gas stream with a liquefied cryogenic fluid. Other examples of heat exchange between liquids and gases, and between gases and gases, are shown in U.S. Pat. Nos. 3,240,262, 3,726,101, 4,027,729, and 3,552,135.
The present invention provides a simple and economical method and apparatus for generating a cold gas, for use in the applications described above, or for other purposes. The apparatus shown provides automatic means of regulating the temperature of the cold gas by regulating the level of cryogenic liquid in a bath.